Re: [PATCH v4 10/21] kvm: x86: Add emulation for IA32_XFD

[Sean Christopherson <seanjc@google.com>]

On Wed, Dec 29, 2021, Yang Zhong wrote:
> From: Jing Liu <jing2.liu@intel.com>
> 
> Intel's eXtended Feature Disable (XFD) feature allows the software
> to dynamically adjust fpstate buffer size for XSAVE features which
> have large state.
> 
> Because fpstate has been expanded for all possible dynamic xstates
> at KVM_SET_CPUID2, emulation of the IA32_XFD MSR is straightforward.
> For write just call fpu_update_guest_xfd() to update the guest fpu
> container once all the sanity checks are passed. For read then
> return the cached value in the container.
> 
> Signed-off-by: Zeng Guang <guang.zeng@intel.com>
> Signed-off-by: Wei Wang <wei.w.wang@intel.com>
> Signed-off-by: Jing Liu <jing2.liu@intel.com>
> Signed-off-by: Yang Zhong <yang.zhong@intel.com>
> ---
>  arch/x86/kvm/x86.c | 27 +++++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index e50e97ac4408..36677b754ac9 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1359,6 +1359,7 @@ static const u32 msrs_to_save_all[] = {
>  	MSR_F15H_PERF_CTL3, MSR_F15H_PERF_CTL4, MSR_F15H_PERF_CTL5,
>  	MSR_F15H_PERF_CTR0, MSR_F15H_PERF_CTR1, MSR_F15H_PERF_CTR2,
>  	MSR_F15H_PERF_CTR3, MSR_F15H_PERF_CTR4, MSR_F15H_PERF_CTR5,
> +	MSR_IA32_XFD,
>  };
>  
>  static u32 msrs_to_save[ARRAY_SIZE(msrs_to_save_all)];
> @@ -3669,6 +3670,19 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  			return 1;
>  		vcpu->arch.msr_misc_features_enables = data;
>  		break;
> +#ifdef CONFIG_X86_64
> +	case MSR_IA32_XFD:
> +		if (!msr_info->host_initiated &&
> +		    !guest_cpuid_has(vcpu, X86_FEATURE_XFD))
> +			return 1;
> +
> +		if (data & ~(XFEATURE_MASK_USER_DYNAMIC &
> +			     vcpu->arch.guest_supported_xcr0))
> +			return 1;
> +
> +		fpu_update_guest_xfd(&vcpu->arch.guest_fpu, data);
> +		break;
> +#endif
>  	default:
>  		if (kvm_pmu_is_valid_msr(vcpu, msr))
>  			return kvm_pmu_set_msr(vcpu, msr_info);
> @@ -3989,6 +4003,15 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  	case MSR_K7_HWCR:
>  		msr_info->data = vcpu->arch.msr_hwcr;
>  		break;
> +#ifdef CONFIG_X86_64
> +	case MSR_IA32_XFD:
> +		if (!msr_info->host_initiated &&
> +		    !guest_cpuid_has(vcpu, X86_FEATURE_XFD))
> +			return 1;
> +
> +		msr_info->data = vcpu->arch.guest_fpu.fpstate->xfd;
> +		break;
> +#endif
>  	default:
>  		if (kvm_pmu_is_valid_msr(vcpu, msr_info->index))
>  			return kvm_pmu_get_msr(vcpu, msr_info);
> @@ -6422,6 +6445,10 @@ static void kvm_init_msr_list(void)
>  			    min(INTEL_PMC_MAX_GENERIC, x86_pmu.num_counters_gp))
>  				continue;
>  			break;
> +		case MSR_IA32_XFD:
> +			if (!kvm_cpu_cap_has(X86_FEATURE_XFD))
> +				continue;

I suspect the 32-bit host support is wrong.  The kernel's handle_xfd_event()
checks for 64-bit support in addition to the CPU feature itself, which implies
that the feature can be reported in boot_cpu_data for 32-bit kernels.

  static bool handle_xfd_event(struct pt_regs *regs)
  {
	u64 xfd_err;
	int err;

	if (!IS_ENABLED(CONFIG_X86_64) || !cpu_feature_enabled(X86_FEATURE_XFD))
		return false;

	...
  }

In this specific case, that means KVM will tell userspace it needs to mgirate
MSR_IA32_XFD, and then reject attempts to read/write the MSR.

If 32-bit host kernels do not explicitly suppress X86_FEATURE_XFD, then KVM needs
to do:

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 556555537a18..156ce332d55b 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -455,9 +455,11 @@ void kvm_set_cpu_caps(void)
 #ifdef CONFIG_X86_64
        unsigned int f_gbpages = F(GBPAGES);
        unsigned int f_lm = F(LM);
+       unsigned int f_xfd = F(XFD);
 #else
        unsigned int f_gbpages = 0;
        unsigned int f_lm = 0;
+       unsigned int f_xfd = 0;
 #endif
        memset(kvm_cpu_caps, 0, sizeof(kvm_cpu_caps));

@@ -545,7 +547,7 @@ void kvm_set_cpu_caps(void)
        );

        kvm_cpu_cap_mask(CPUID_D_1_EAX,
-               F(XSAVEOPT) | F(XSAVEC) | F(XGETBV1) | F(XSAVES) | F(XFD)
+               F(XSAVEOPT) | F(XSAVEC) | F(XGETBV1) | F(XSAVES) | f_xfd
        );

        kvm_cpu_cap_init_scattered(CPUID_12_EAX,

> +			break;
>  		default:
>  			break;
>  		}